1. Field of the Invention
The present invention relates to a magnetic field generator including a plurality of magnetic field generation units, and to a magnetic sensor system and a magnetic sensor each including the magnetic field generator.
2. Description of the Related Art
In recent years, magnetic sensor systems have been employed to detect a physical quantity associated with the rotational movement or linear movement of a moving object in a variety of applications. U.S. Patent Application Publication No. 2014/0292322 A1 discloses a magnetic sensor system that includes a scale and a magnetic sensor and is configured so that the magnetic sensor generates a signal associated with the relative positional relationship between the scale and the magnetic sensor.
The magnetic sensor includes a magnetic detection element for detecting a magnetic field to be detected. Hereinafter, the magnetic field to be detected will be referred to as the target magnetic field. U.S. Patent Application Publication No. 2014/0292322 A1 discloses a magnetic sensor that uses a so-called spin-valve magnetoresistance (MR) element as the magnetic detection element. The spin-valve MR element includes a magnetization pinned layer having a magnetization pinned in a certain direction, a free layer having a magnetization that varies depending on the target magnetic field, and a nonmagnetic layer located between the magnetization pinned layer and the free layer. Examples of the spin-valve MR element include a TMR element in which the nonmagnetic layer is a tunnel barrier layer, and a GMR element in which the nonmagnetic layer is a nonmagnetic conductive layer.
The scale of the magnetic sensor system includes a plurality of magnetic field generation units arranged in a predetermined pattern to generate a plurality of external magnetic fields. Typically, each of the plurality of magnetic field generation units is formed of a permanent magnet. The plurality of magnetic field generation units are magnetized in alternating directions. This causes the external magnetic fields generated by the plurality of magnetic field generation units to be in alternating directions.
Some magnetic sensors have means for applying a bias magnetic field to the magnetic detection element. The bias magnetic field is used to allow the magnetic detection element to respond linearly to a variation in the strength of the target magnetic field. In a magnetic sensor that uses a spin-valve MR element, the bias magnetic field is used also to make the free layer have a single magnetic domain and to orient the magnetization of the free layer in a certain direction, when there is no target magnetic field.
U.S. Patent Application Publication No. 2014/0292322 A1 discloses a magnetic sensor including a bias magnetic field generator for generating a plurality of bias magnetic fields to be applied to a plurality of MR elements. The bias magnetic field generator includes a plurality of pairs of magnetic field generation units provided in correspondence with the plurality of MR elements. Every two magnetic field generation units pairing up with each other are arranged with a corresponding one of the MR elements in between. Each magnetic field generation unit is formed of a permanent magnet and generates an external magnetic field.
A structure including a plurality of magnetic field generation units arranged in a predetermined pattern to generate a plurality of external magnetic fields, such as a scale, will hereinafter be referred to as magnetic field generator. In a magnetic sensor including a bias magnetic field generator, a plurality of magnetic field generation units constituting the bias magnetic field generator are arranged in a predetermined pattern. Thus, the bias magnetic field generator can also be said to be a magnetic field generator.
Magnetic sensor systems and magnetic sensors each including a magnetic field generator that includes a plurality of magnetic field generation units each formed of a permanent magnet suffer from the following problem. Such magnetic sensor systems and magnetic sensors are typically used under the condition that the strength of the target magnetic field does not exceed the coercivity of the permanent magnets. However, since the magnetic sensor systems and the magnetic sensors can be used in various environments, an external magnetic field having a strength exceeding the coercivity of the permanent magnets can happen to be temporarily applied to the permanent magnets. When such an external magnetic field is temporarily applied to the permanent magnets, the magnetization direction of the permanent magnets may be changed from an original direction and then remain different from the original direction even after the external magnetic field disappears. In such a case, the direction of the magnetic field generated by each magnetic field generation unit changes to become different from a desired direction.
Further, the magnetic field generator including a plurality of magnetic field generation units each formed of a permanent magnet has a problem in that the plurality of magnetic field generation units are difficult to arrange in a desired pattern. This problem will be described in detail below by taking as an example a magnetic field generator in which the plurality of magnetic field generation units are magnetized in alternating directions, such as a scale. The following description assumes that the magnetization directions of the plurality of magnetic field generation units alternate between a first direction and a second direction. A plurality of magnetic field generation units magnetized in the first direction will be referred to as a plurality of first magnetic field generation units. A plurality of magnetic field generation units magnetized in the second direction will be referred to as a plurality of second magnetic field generation units. This magnetic field generator is fabricated by the following method.
First, an initial magnetic field generator including a plurality of initial magnetic field generation units that are not magnetized in a predetermined direction is fabricated. Next, a plurality of ones of the initial magnetic field generation units that are intended to become a plurality of first magnetic field generation units are subjected to a magnetic field in the first direction having a higher strength than the coercivity of those plurality of ones of the initial magnetic field generation units, whereby those plurality of ones of the initial magnetic field generation units are magnetized in the first direction. At this time, the other plurality of ones of the initial magnetic field generation units that are intended to become a plurality of second magnetic field generation units are not subjected to any magnetic field having a higher strength than the coercivity of those initial magnetic field generation units. The initial magnetic field generation units magnetized in the first direction become the plurality of first magnetic field generation units.
Next, the initial magnetic field generation units intended to become the plurality of second magnetic field generation units are subjected to a magnetic field in the second direction having a higher strength than the coercivity of those initial magnetic field generation units, whereby those initial magnetic field generation units are magnetized in the second direction. At this time, the plurality of first magnetic field generation units, which have already been magnetized in the first direction, are not subjected to any magnetic field having a higher strength than the coercivity thereof. The initial magnetic field generation units magnetized in the second direction become the plurality of second magnetic field generation units.
The foregoing fabrication method for the magnetic field generator requires that the magnetic fields to be applied have strengths largely different between two adjacent initial magnetic field generation units or between one of the first magnetic field generation units and one of the initial magnetic field generation units adjacent each other. To achieve this, measures such as an increase in the distance between the two adjacent magnetic field generation units are required. Thus, in the magnetic field generator including a plurality of magnetic field generation units each formed of a permanent magnet, the plurality of magnetic field generation units are difficult to arrange in a desired pattern.
An increase in the distance between two adjacent magnetic field generation units results in a reduction in flexibility in the arrangement of the plurality of magnetic field generation units and an increase in the area occupied by the plurality of magnetic field generation units. Furthermore, the difference of the external magnetic fields is dull between the two adjacent magnetic field generation units, thus causing a reduction in the resolution of the magnetic sensor system using the magnetic field generator as the scale.